With the spread of the Internet, IP telephone services using the Internet are being provided by telecommunications carriers and the Internet Service Providers (ISPs). Network forms in which analog and ISDN telephone lines are connected to IP networks by use of Media Gateways (MGs) are also spreading. IP telephone services provide Voice-over-IP (VoIP) communications in which voice data is transferred in Real-time Transport Protocol (RTP) packets by establishing channels (sessions) between terminals before communications start.
As a session control protocol for establishing and disconnecting sessions between terminals, Session Initiation Protocol (SIP) is defined, in which a session is controlled between terminals through an SIP server.
An SIP message includes a start line field and a header field for carrying a request or response information and a message body field for describing contents of a session. For example, a Session Description Protocol (SDP) is applied to describe contents of a session of an SIP message. In an SIP message, a communication party is identified using a request Uniform Resource Identifier (URI) described in the start line field. Additionally, in the SIP message, RTP packet communication conditions between terminals such as protocols of, e.g., a call for a callee, voice, and video and a bit rate are defined in the process of establishment of a session.
With the spread of the Internet, unwanted e-mails transmittable substantially without cost are increasing. This is problem in society. A large number of unwanted e-mails are transmitted to mail addresses collected from bulletin boards and websites on the Internet and anticipated mail addresses. This is a burden on mail servers. For example, malicious unwanted e-mails use fake source addresses.
Like these unwanted e-mails, there is a concern that a large number of unwanted calls using fake sources may be transmitted to collected and anticipated telephone numbers. In the past, unwanted calls were performed by persons. However, SPAM over IP Telephony (SPIT) using Internet Protocols (IPs) is generated by automatically running SPAM generation software by use of a computer. Basically, in IP telephone services, when a caller requests connection to a callee, a ring tone is played. Therefore, there is a possibility that SPIT may be constantly received as unwanted calls. Even when unwanted e-mails are received, the e-mails are first stored in a server and can be deleted without reading them. However, since a ring tone is played in response to each reception of an unwanted e-mail, there is a possibility that a callee may suffer from the ring tone 24 hours a day, throughout the year. Therefore, the problem of unwanted calls is more serious than that of unwanted e-mails.
Communications with IP phones are also possible via MGs over analog and ISDN telephone lines. Therefore, the same concern arises.
There are examples in which an automatic voice response is provided to prompt key inputs, e.g., for selecting services in an answer machine, a facsimile phone, and a call center. However, to provide services in these examples, predetermined keys are announced by the automatic voice response and inputs of the keys are received. Therefore, there is a possibility that the pattern may be easily learned and unwanted calls of SPIT may be received.
In JP-A No. 2007-006467, a Turing test is performed to determine whether a caller transmits an unwanted call of SPIT by providing an SPIT filter between a caller and callee. In a Turing test using the SPIT filter, the determination is performed by comparing timing of response to questions by an automatic response with an expected value of answers to the questions.
In JP-A No. 2007-006467, however, the response timing is determined using a threshold and signal energy of the response. Additionally, no clear answer to be compared is shown in the determination using an expected value of the answers. Accordingly, when ambiguous answers cause false positives, SPIT bypasses the test and is then received. Additionally, multiple answer samples for automatic determination, sampling of signal energy, etc. need a high processing ability and a large amount of memory. Further, an ambiguous answer causes a multistage test, which bothers a caller.